Dissolution of Silk Fibroin in Mixtures of Ionic Liquids and Dimethyl Sulfoxide: On the Relative Importance of Temperature and Binary Solvent Composition

Omar A El Seoud; Marc Kostag; Shirley Possidonio; Marcella T Dignani; Paulo A R Pires; Matheus C Lourenço

doi:10.3390/polym14010013

Dissolution of Silk Fibroin in Mixtures of Ionic Liquids and Dimethyl Sulfoxide: On the Relative Importance of Temperature and Binary Solvent Composition

Polymers (Basel). 2021 Dec 21;14(1):13. doi: 10.3390/polym14010013.

Authors

Omar A El Seoud¹, Marc Kostag¹, Shirley Possidonio², Marcella T Dignani¹, Paulo A R Pires¹, Matheus C Lourenço¹

Affiliations

¹ Institute of Chemistry, The University of São Paulo, Sao Paulo 05508-000, Brazil.
² Department of Chemistry, Institute of Environmental, Chemical, and Pharmaceutical Sciences, Federal University of São Paulo, Sao Paulo 04021-001, Brazil.

Abstract

We studied the dependence of dissolution of silk fibroin (SF) in mixtures of DMSO with ionic liquids (ILs) on the temperature (T = 40 to 80 °C) and DMSO mole fraction (χ_DMSO = 0.5 to 0.9). The ILs included BuMeImAcO, C₃OMeImAcO, AlBzMe₂NAcO, and Bu₄NAcO; see the names and structures below. We used design of experiments (DOE) to determine the dependence of mass fraction of dissolved SF (SF-m%) on T and χ_DMSO. We successfully employed a second-order polynomial to fit the biopolymer dissolution data. The resulting regression coefficients showed that the dissolution of SF in BuMeImAcO-DMSO and C₃OMeImAcO-DMSO is more sensitive to variation of T than of χ_DMSO; the inverse is observed for the quaternary ammonium ILs. Using BuMeImAcO, AlBzMe₂NAcO, and molecular dynamics simulations, we attribute the difference in IL efficiency to stronger SF-IL hydrogen bonding with the former IL, which is coupled with the difference in the molecular volumes and the rigidity of the phenyl ring of the latter IL. The order of SF dissolution is BuMeImAcO-DMSO > C₃OMeImAcO-DMSO; this was attributed to the formation of intramolecular H-bonding between the ether oxygen in the side chain of the latter IL and the relatively acidic hydrogens of the imidazolium cation. Using DOE, we were able to predict values of SF-m%; this is satisfactory and important because it results in economy of labor, time, and material.

Keywords: design of experiments; effects of temperature; ionic liquid molecular structure; molecular dynamics simulations; silk fibroin dissolution.